1. Field of the Invention
The present invention is directed to a device for electrochemically measuring the corrosion rate of a reinforced steel bar used in concrete structures, by non-destructive methods, with which a high measuring accuracy is obtained.
It is also an object of the invention to provide corrosion detecting probes for use with the device of the present invention.
2. Description of the Prior Art
Concrete is the most frequently used building material for all kinds of structures, such as houses, bridges, roads, dams, irrigation ditches, etc. The steel reinforcements embedded in these concrete structures remain indefinitely free of corrosion unless the concrete contains chlorides or it reacts with atmosphere carbon dioxide.
When the metal reinforcement corrodes, the rust build up breaks the concrete cover and this loss of physical integrity jeopardizes the structural integrity and load bearing capacity of the concrete structure.
Thus, diagnosis in situ of the metal structure corrosion rate is of high economic interest.
Leaving aside destructive methods, that is, those in which measurements are performed by partially destroying the structure, which cannot in a large majority of cases even be practiced, non-destructive methods are known, based on applying a small amplitude electrical signal to the reinforced steel bar and measuring the response it provokes.
The ratio between the applied potential and the response intensity is inversely proportional to the corrosion rate, and it is known as Polarization Resistance.
Its theoretical basis was given by Stern and other authors starting from 1957. For the value to be quantitative it is necessary that one know the area of a rebar (reinforced steel bar) to which the applied potential is being applied; thus can be calculated ampere/unit area values in the manner summarized in the article by S. G. McKenzie entitled, Techniques for Monitoring Corrosion of Steel in Concrete, Corrosion Prevention & Control, February, 1987, pg 1. In that article, the author emphasizes the importance of knowing the "area really affected by the applied signal". However, the author states that the direct use of the Apparent Polarization Resistance values does not reliably permit one to measure an on-site corrosion rate. McKenzie does not explain how to resolve this problem but only stresses the need to resolve it.
While conventional non-destructive methods as described in the McKenzie article offer acceptable results at the laboratory level where the extent of the metal area is known, such methods lack reliability in practice when applied to large structures, since for achieving this, it is mandatory to delimit or calculate the rebar area to which a measuring signal is applied.
As stated above, when dealing with real scale structures this problem poses a great obstacle in performing successful measurements. When applying an electrical signal, the signal will spread along the rebar until it reaches a critical length which is a function of the corrosion rate itself, of the concrete's water content (i.e., its resistivity), of the metal reinforcement surface, and of concrete thickness.
In order to overcome the above difficulty, different solutions have been tried and patented including U.S. Pat. No. 4,958,130 (18 Sep. 1990) and U.S. Pat. No. 4,861,453 (29 Aug. 1989), which disclose various devices, detecting probes and electrode arrangements for calculating True Polarization Resistance.
U.S. Pat. No. 4,958,130 discloses a method based essentially on measuring simultaneously three parameters: corrosion potential, polarization resistance and concrete ohmic resistance. These measurements are taken over in a plurality of points, and are functionally related to the concrete surface, the diameter of a reinforced steel bar, a sensor diameter and the gap between adjacent reinforced steel bars. The inventors claim they are able to obtain accurate values of True Polarization Resistance, however, no claim is made as to any specific corrosion detecting probes.
U.S. Pat. No. 4,861,453 discloses a corrosion detecting probe having a central counter and reference electrodes and another annular counter electrode, assembled in a material filled with a particular electrolyte which is electrically connected to a measuring device.
None of these references resolve accurately the calculation of a rebar area actually affected by an applied potential signal because:
a) the use of alternating current, even in the case where a guard ring is used, does not provide accurate values of the Polarization Resistance or does not allow an efficient confinement;
b) the calculations or methodology used are not well explained, nor do the references clarify or overcome the difficulty of making reliable measurements of the applied potential as it extends along a reinforcing bar.